The way I usually create custom linetypes, I compile the SHP file into a SHX file. And I include the shapefile extension in the linetype definition.
What happens if you compile your SHP file into SHX?
You might also need to change your definition:
*PARKINGBAY, 2.5 x 4.5 parking bay
A,1.25,-0.0,[MYLINE,AECCLAND.SHX,S=4.5,Y=-4.5],1.25
All of my linetypes (including the ones created by Autodesk) use the SHX extension in the linetype definition. I'm not sure what Acad does if that extension is missing.
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Sinc